Nickel-base alloys adaptable for use as steam turbine structural components

ABSTRACT

Nickel-base alloys containing correlated amounts of aluminum, titanium, columbium, molybdenum and tungsten, adaptable for use as structural components (notably steam turbine bolts) at temperatures on the order of 1000* F. Alloys, which also contain iron and usually carbon, afford specified minimum room and elevated temperature yield strengths and other desirable metallurgical characteristics.

United States Patent on 3,619,183

72 Inventors John H. Olson [56] Relerences Clted Franklin Lakes N44UNITED STATES PATENTS I 2]] No 2,994,605 8/l96l can et al. 75/171 FiledMar I968 3,046JO8 7/l962 lusclstcm 75/! 7| [45] Pat nt d Nov, 9, 1971Primary ExaminrrRlchard 0. Dean [73] Assignee The lnternatlonal NickelCompany, Inc. L Pin l New York, NY.

[54] ABSTRACT: Nickel-base alloys containing correlated SC I N D Iamounts of aluminum, titanium. columbium. molybdenum o and tungsten.adaptable for use as structural components [52] US. Cl 75/l7l. (notablysteam turbine bolts) at temperatures on the order of l48/32.5. l48/l62l000 F. Alloys, which also contain iron and usually carbon [5]] lnt.C|C221: 19/00 afford specified minimum room and elevated temperature [50]Field of Search 75/] H, yield strengths and other desirablemetallurgical characteristics.

NICKEL-BASE ALLOYS ADAPTABLE FOR USE AS STEAM TURBINE STRUCTURALCOMPONENTS As is generally known, nickel-base alloys have foundconsiderable acceptance in innumerable commercial and industrialapplications, applications requiring adherence to critical servicerequirements. For example, such materials have been gainfully utilizedin the production of various components for steam turbine environments,including both rotors and blades. Nonetheless and as is so often thecase, advanced structural designs brought about by increasingly severeoperating conditions necessitate the development of new alloys capableof withstanding the more stringent demands imposed. Steam turbinebolting is illustrative.

Not long ago the 12 percent chromium stainless steels were extensively,if not exclusively, used as the bolting material in fastening the outershell sections of steam turbine assemblies. But with the introduction oflarger size turbines designed to operate at temperatures circa l,000 F.such steels were deemed wanting, particularly in respect of strengthcapabilities. In this regard and as contemplated herein, bolting alloysshould be capable of consistently delivering in the aged condition aminimum room temperature yield strength (0.02 percent offset) of atleast 85,000 pounds per square inch (p.s.i.) and a l000 F. minimum yieldstrength of better than 70,000 p.s.i. And provided other desiredproperties are not sacrificed, it is most beneficial that these minimabe at least 90,000 p.s.i., and 75,000 p.s.i., respectively. 1

One of the currently used bolting materials has on occasion manifestednotch sensitivity," i.e., its load carrying ability is considerablyimpaired by the presence of a notch. As a practical matter, it isvirtually impossible to absolutely preclude the formation of a notch,crack, or similar flaw, flaws which may be internally or externallyinduced either during or subsequent to processing. Once present, a flawostensibly serves as a focal point for concentration of stresses. Thislocalization of stress concentration promotes the notch as a point ofself-propagtion. How well a material resists this propagation is areflection of its notch sensitivity or notch toughness.

A drawback of the above-mentioned notch sensitive alloy concerns thecoefiicient of thermal expansion (CTE) parameter. Usually the turbineshell is formed from a ferritic steel having a CTE of between about7.75Xl0 to 8Xl0 in./in./ F. at the operating temperature of l,000 F.;however, at such temperature this notch sensitive alloy has a CTE on theorder of about 8.4Xl0" in./in./ F. Quite naturally, a bolting alloyshould exhibit a CTE as compatible as practicable with the shellmaterial and to this end a CTE of below about 8 would be of considerableadvantage. Steam turbines are often "torn down for-purposes ofinspection, repair, etc. On cooling the bolts (and nuts) tend tocontract and thus tighten. At best the bolt is difficulty removable, andthe wider the gap between the CTEs of shell and bolt the more acute andtedious the problem becomes.

In addition to the foregoing, alloys suitable for steam turbine bolting(or other high temperature fasteners) should also manifest a low creeprate at temperatures of about l,000 F. to l,200 F., otherwise, should abolt undergo substantial deformation (extension) in use, seriousconsequences could ensue. More specifically, such alloys should becharacterized by high resistance to what is commonly termed relaxation."As treated in excellent fashion at page 604 of the Eighth Edition of THEMETALS HANDBOOK, turbine bolts are tightened initially to a certainelastic strain and corresponding elastic stress. While in service atelevated temperature, bolt creep does occur. It is considered that someportion of elastic strain is thus transformed into plastic strain. As aresult, there is reduction in stress and this is given the aptdescription relaxation. lf the degree of relaxation is too much, whatwas a leakproof joint is no longer.

Too, it would be particularly desirable if the same alloys developed forbolting were characterized by good stress rupture properties attemperatures of l,000 F. to 1,200 F including stress rupture strength,ductility and resistance to impact such that the alloys would be usefulfor other applica-- tions, e.g., tubing, piping, valves, and the like.The subject invention is addressed to attaining these objectives whileobviating difficulties heretofore experienced.

It has now been discovered that nickel-base alloys of specialcomposition and containing interrelated amounts of aluminum, titanium,columbium, molybdenum, tungsten. etc., display an excellent combinationof room and elevated temperature characteristics rendering the alloysparticularly useful in the production of various articles of utility,particularly steam turbine bolts.

It is an object of this invention to provide a new and improvednickel-base alloy.

Another object is to provide nickel-base alloys capable of manifestingyield strengths of at least 85,000 p.s.i. at room temperature and of atleast 70,000 p.s.i. at l,000 F the alloys also being substantially notchinsensitive and characterized by good ductility.

Other objects and advantages will become apparent from the followingdescription.

Generally speaking, alloys contemplated herein are age hardenable and,in accordance with the invention, contain (percent by weight) from 17.5percent to 22 percent chromium, about 2.3 percent to 3.3 percent ofcolumbium, about 2.5 percent to about 3 percent molybdenum, about 2.5percent to about 3.25 percent tungsten, about 0.4 percent to about 0.75percent aluminum, about 0.35 percent to about 0.7 percent titanium, upto about 0.l2 percent e.g., about 0.01 percent (advantageously 0.04percent) to 0.l percent carbon, about 3 percent to about 12 percent,e.g., 5 percent to 9 percent, iron, up to about 0.1 percent, e.g., 0.0lpercent to 0.05 percent, magnesium, up to about 0.01 percent, e.g.,0.003 percent to 0.008 percent, boron, up to 0.l percent, e.g., 0.0]percent to 0.05 percent, zirconium, up to about 0.4 percent silicon, upto 0.75 percent manganese, and the balance essentially nickel. The useof the expression balance" or essentially" in referring to the nickelcontent of the alloys, as will be understood by those skilled in theart, does not exclude the presence of other elements commonly present asincidental constituents, e.g., deoxidizing and cleansing elements, andimpurities normally associated therewith in small amounts which do notadversely affect the basic characteristics of the alloys. In thisregard, elements such as oxygen, nitrogen, phosphorus, sulfur, and thelike should be kept as low as is practicable. Tantalum, as is known, isusually found in commercially produced columbium and can be present, forexample, up to 0.5 percent.

In carrying the invention into practice, it is important in consistentlyachieving highly satisfactory results on a production basis that thechromium content not fall below 17.5 percent, for, as will beillustrated herein, inadequate tensile strength can easily be theresult. The efiect of chromium in this regard is deemed somewhatsurprising. Usually, chromium is considered to perform as a simple solidsolutioning element without much effect on age hardening response.Although a complete theoretical explanation may not yet be at hand,chromium in accordance herewith, appears to impart a rather stronginfluence on the age hardening characteristics of the alloy. Whateverits exact role, it markedly contributes to achieving the necessarystrength levels at both room and elevated temperatures. There arecertain instances in which chromium can be lowered to about 15 percent,possibly l4 percent, as will be illustrated herein. But this opens up anarea of risk. In any event, however, percentages above 22 percent are tobe avoided lest alloy stability be impaired. In consistently achieving asufficient strength plateau and with the view of minimizing loss ofstability, it is, accordingly, quite beneficial that the alloys containnot less than l9 percent nor more than 21 percent chromium.

Aluminum, even in the small amounts contemplated, exerts a most potentinfluence in imparting hardness and in conferring high tensile strength(at both room and elevated temperature) and stress-rupture strength.Reducing the aluminum much below 0.4 percent entails the objectionablerisk of inadequate tensile strength. At the other end of the range,amounts much above 0.75 percent can undesirably detract from stressrupture ductility characteristics, particularly when the other essentialconstituents, columbium, titanium, molybdenum, tungsten, are at thehigher end of their respective ranges. Moreover, the higher levels maygive rise to an additional complicating factor. By way of explanation,alloys within the invention upon exposure to high temperature forextended periods of time tend to exhibit an increase in yield strength.This is deemed attributable to additional age hardening precipitatecoming out of solution upon exposure to high temperature. Now, there maybe applications in which this may not be objectionable, but for steamturbine bolting dimensional change might result with attendantdifficulties. Thus, the mere presence of excess aluminum could possiblyaccentuate this condition, i.e., increase the possibility for additionalage hardening constituent to come out of solution upon prolonged heating(and subsequent to usual age hardening treatments). While this affectcan be minimized by special heat treatment, as will be discussedhereinafter, control of aluminum content is of advantage.

ln addition to aluminum, care must also be exercised in respect of theamounts of titanium, columbium, molybdenum, and tungsten. Titanium, forexample, contributes'to tensile strength and hardness and also improvesstress rupture life, although to a lesser degree than aluminum. Unlikealuminum, however, it significantly impairs alloy stability as evidentfrom a not insubstantial loss in the capability of absorbing impactenergy upon prolonged exposure to elevated temperature. For bestresults, including yield strength, stability and high temperatureductility, neither the lower nor the upper limits, respectively, of bothof these constituents should be used simultaneously. In this regard, thesum of the aluminum plus titanium should be at least above 0.9 percent,advantageously at least 1 percent, and up to 1.4 percent.

Columbium, molybdenum and tungsten coact to confer hardness andstrength. In addition, columbium enhances stress rupture life, butlowers stability, particularly in conjunction with molybdenum. In termsof loss in impact strength the copresence of columbium and molybdenum issynergistic in effect, the loss being greater than what it might be forthese constituents individually. Accordingly, with the desideratum ofreaching the best combination of strength and stability the combinedcolumbium plus molybdenum should not exceed about 6 percent. Tungsten,although it exerts a positive influence in terms of strength, whenpresent to the excess detrimentally affects high temperature ductilityand undesirably raises alloy density. In view thereof and for highlysatisfactory results, it beneficially should not exceed about 3 percent.

As a result of test data, it has been'determined that the total amountof the hardening and strengthening constituents (aluminum, titanium,columbium, molybdenum and tungsten) should not be concomitantly used atthe upper end of their respective ranges if acceptable levels of bothalloy stability and ductility are to be attained. On the other hand,there is the danger that if the minima of each of such elements areused, insufficient strength will result. Accordingly, these constituentsshould be correlated such that the strengthening and stability factor,SSF, expressed by the following relationship is satisfied:

about 5.25% to 6.4%.

Advantageously the SSF is from 5.4 percent to 6.2 percent.

A most highly satisfactory combination of properties is obtained withthe alloys falling within the following ranges: about 19 percent to 21percent chromium, about 2.5 percent to 3 percent each of columbium,molybdenum, and tungsten, about 0.5 percent to 0.7 percent aluminum,about 0.4 percent to 0.6 percent titanium, the sum of the aluminum plustitanium being at least 1 percent, about 0.04 percent to about 0.1percent carbon, about 5 percent to 9 percent iron, about 0.003 percentto 0.008 percent boron, about 0.01 percent to 0.05 percent zirconium,and the balance essentially nickel.

Conventional processing techniques can be used in producing the alloys,but to achieve acceptable strength, they should be aged at a temperatureof about l300F. to l400F. for a period of about 16 to 32 hours, e.g.1300+F. for about 24 hours. To minimize the occurrence of a quitesubstantial increase in yield strength when the alloys are exposed tohigh temperature for periods of long duration, an additional aging stepis deemed quite beneficial. When aged over the temperature range of1300F. to l400F.(even for the considerable period of, say, 32 hours) andthereafter subjected to exposure at a temperature of about l000F. for anextended period of time, e.g. 1000 hours, it has been observed that theyield strength rather markedly increases. This behavior, as mentionedabove herein, ostensibly is due to a greater amount of precipitatinghardening phase coming out of solution whereby strength is increased.Since this might give rise to dimensional change, a second agingtreatment is recommended at least for alloys intended for boltingapplications. Thus, subsequent to the first aging treatment above setforth the alloys should be cooled to a temperature of 1,l00 F. to l,200F. at a rate of about 20 F to 50 F. per hour and then held for about 15to 30 hours. As a result of the second aging treatment, the percentageincrease in yield strength that might otherwise be expected isconsiderably reduced. This, in turn, minimizes the possibility ofdimensional change.

For the purpose of giving those skilled in the art a better appreciationof the invention, the following illustrative data are given:

A series of alloys both within and without the invention were preparedusing vacuum melting techniques. The melt charges were made using thefollowing type of ingredients: carbonyl nickel pellets, vacuum gradechromium, electrolytic iron, ferro-columbium, molybdenum pellets,tungsten powder, titanium sponge, aluminum rod, ferro-boron, andspectrographic carbon. Magnesium and zirconium were introduced in theform of nickel-magnesium and nickel-zirconium master alloys,respectively. Generally, the major charge components with about 0.05percent carbon were melted in a magnesium oxide crucible and held aboutone-half hour at about 2,900 F. to effect oxygen removal. Additions oftitanium, aluminum, boron, and zirconium were made and argon wasintroduced into the vacuum chamber (one-half atmosphere pressure)following which the nickel-magnesium master alloy was added. A finalcarbon addition was next made and ingots (30 lbs.) were thereafterpoured at 2,850 F.

Usually the ingots were soaked at least 1 hour at 2,l50 F. and hotrolled to 2inches X 2 inches bar which was thereafter annealed at 2,l50F. and hot rolled to 96-inch square bar using one intermediate anneal atabout 2,l50 F. In some instances this exact practice was not followedduring the early stages of the rolling. lngots which were not rolleddown to 2 inches X 2 inches bar during the initial hot rolling werehammer-forged to produce stock that could be rolled further. In anycase, it was considered that the final hot rolling procedures weresimilar for all heats, the average finishing temperature being estimatedto be 1,750 F.

In table l, alloys 1 through 8 represent alloys formulated in accordancewith the invention whereas alloys A through H are beyond the scopethereof. These later alloys, however, afford a good basis for comparisonwith alloys 1 through 8 in terms of mechanical characteristics, theresults being reported in table 11. Too, it should be noted that alloys7 and 8 are included to demonstrate that it is possible to obtain anacceptable level of properties with alloys containing chromiumpercentages of about I5 percent to 16 percent, i.e., below 17.5 percentor 19 percent. The mechanical characteristics were obtained on specimenswhich had been annealed at l,800, F. for 1 hour and thereafter aged forabout 24 hours at l,300 F. (Apart from the chemistry given in table I,each of the alloys contained about 5 percent to 9 percent iron, not morethan (a) 0.04 percent zirconium, (b) 0.0l percent boron, (c) 0.03percent magnesium, (d) 0.1 percent manganese, (e) 0.1 percent ployed asreflected by alloys 7 and 8. In this connection. the silicon, and (f)0.31 percent tantalum, the balance being esaluminum content must be atleast 0.5 percent and adsentially nickel and impurities.) vantageouslyat least 0.6 percent. the sum of the aluminum plus titanium being atleast l.l percent, e.g., 1.2 percent. This 5 further restricts analready narrow aluminum range. Since TABLE 1. (OMlUSlTlON somepercentage of aluminum is lost during processing, it thus can becomedifficult (with attendant risk) to consistentl y 8.3 1. achieve thenecessary minimum in strength. etc. -lac- Ni. Cr Ch Ti Al Mo w Immm mStress rupture tests were also conducted on \JHOUS alloys, the testsbein carried out usin combination t e s ecimens, g 8 VP P lfg. i.e.,specimens having two reduced sections, one smooth and 20 3.0 0.60 0.72.55 '2. 0. 8 0. 001 31-1 one notched, the notched being a 0.007-inchroot radius. a 1 51;: 01 01 :12; 2:0 3:8 00:2 21:; 11:; Th s prim ry f rh 0 th purp s r 20.0 0. 37 0.68 8.8311 B ascertaining notch sensitivitycharacteristics. The results are 1.. '...i' 3: 3 332 3 72 n3 given intable lll and it should be mentioned that the 0-065 1 s ecimens wereroom tem erature tensile tested after ex 0- P P P s..... 15.4 3.1 0. 500.50 2. .10 2. 0.085 5. .1. 110. a o .1. 3.15 0. as 0.10 2.50 a. 0 0.105 11 110. sure at 1,000 F. or 1,200 F. (Alloy Zhavmg been exposed at 3g?both temperatures). The time of exposure and stress are also I v t 11 I1).... 15.8 3.0 0. 00 0.11 2.5 3.0 0.003 5. 311 1:0. 20 given togetherwith room temperature yield strength (Y.S.), 16.1 2.45 0.36 0.50 3.852.55 0.086 5.31 110. M6 305 [L53 0'48 195 0' 11 5.64 no tensileelongation (El.), reduction of area (RA notch ten (1.... 14.0 3.0 0. 520. 43 3.1.- 3.: 0.017 5.00 110. Sll strength(N.T.S.), and the ratioofnotch tensile strength to r- 051 ultimate tensile strength (NTS/UTS).

TABLE III Stress rupture conditions Smooth liar, percent 'lcm- Yivl ll lI 1 perm streii t i E (111- lvr l1t- Tiiin turv. 10.0. cation tioit llNTS/ l10iii- 1 p.s.i. ttt'tfl NTS UTS Alloy No.1

X0111 X0110 Noiit- 103,100 33 5 (None X0110 X0110 8 7,500 11." "1 1,00015, 000 1,200 107,700

TABLE It The results depicted in table lll illustrate that alloys withinYield Strength Elongation, Reduction in the invention are notchinsensitive. Actually, where failure ocpercent area. pe curred, it wasin the smooth portion of the test specimen and Rm 1,000 D R313 R311.L000 under a stress quite above that normally encountered in ser- A Nvice for steam turbine bolting. By way of explanation and with Wreference to the data in table Ill, no failure was ex erienced withalloy 2 even after 1,800 hours exposure at l,O00 F. under theexceptionally high stress of 100,000 p.s.i., failure not occurring untilanother specimen was tested at l,200 F. In this latter instance, failurecame about at 190 hours exposure at the still very high stress of 80,000p.s.i. The failure that l5. did occur was ductile in manner, theelongation being'37 perg: cent and the reduction of area being 62percent. The notched E. portion of this specimen was put back in testand was exposed for an additional 1,502 hours (making a total of 1,692hours) 11... whereupon the test was discontinued. Subsequent testingrevealed the specimen was not weakened as a consequence of 0 the notchsince the room temperature tensile test showed the n be noted from thedata given in tables I and H that ultimate tensile strength to bel89,900 p.s.i. versus the each of the alloys within the subjectinvention manifested 167,500 .s.i. obtained in the initial agedcondition. yield strengths at room temperature in excess of the requiredA third specimen of alloy 2 was exposed at the higher temminima of85,000 p.s.i. at room temperature and 70,000 p.s.i. per-alum f 1,200 F.but nd a mor re li ti tres of at l,000 F. ln m rke Con ra h r t n Of theoy 65 55,000 p.s.i. No failure followed the exposure period of 1,248Sidelhe invention afforded y Strengths fy g hours and the test wasdiscontinued. Nor was failure exthe same minimum requirem nts. A oys Aand 3 gh perienced in respect ofalloys 4 and 5 after exposure periods ofsufficiently high in chromium exhibited inferior strength due 1050 and1,000 hours, respectively, at l,200 F. (tests disconto the low total ofaluminum plus titanium. Also, the Strengthtinned). Stability Factor,SSF, was low. On the other hand, alloys such As referred to herein, asecond aging treatment is beneficial as D through H having an adequatetotal of aluminum plus in minimizing the tendency for the alloys toexhibit increased titanium were still unsatisfactory, the reason beingattributed ield strength upon prolonged exposure at elevated temperatolow chromium levels. With higher aluminum contents and t re. Thus, toillustrate this aspect, alloy 2 was double aged, an SSF of at least 5.5,beneficially at least 5.7, alloys containi.e., after aging at 1,300 F.for2 4 hours, it was furnace cooled ing down to 15 percent or 14 percentchromium can be emto 1.150" F. at a rate of about 25.20 F. per hour andheld (aged) for 24 hours. After this second aging treatment, alloy 2exhibited a yield strength (0.02 percent offset) of 118,000 p.s.i.,approximately 15,000 p.s.i. above that obtained with the single agingtreatment conducted for 24 hours at l,300 F. This value of 1 18,300p.s.i. represents a reduction of one-half in the difference in yieldstrength obtained prior and subsequent to long time exposure. Thedifferential in yield strength generally would not be expected to be asgreat with most other compositions within the invention. Alloy 2 (alsoalloy 1) was the most heavily alloyed in terms of aluminum. titanium.columbium, molybdenum. and tungsten and. thus. would be compositionallymost susceptible to manifest increased strength upon prolonged exposureat elevated temperature.

1n the double aged condition, alloy 2 was tested for notch sensitivityby exposing the alloy for approximately 1,2 hours at l,000 F. under astress of 1 15,000 p.s.i. No failure was encountered whereupon thestress was raised to 125,000 p.s.i. and the test was continued for anadditional 1 19 hours. When failure had not yet occurred, the stress wasraised to 135,000 p.s.i., failure being brought about after 95.4 hoursat temperature. Again failure was experienced in the smooth section.This cycle of testing confirmed that the alloys within the inventionexhibited excellent resistance to notch sensitivity.

As indicated above herein, the coefficient of thermal expansion foralloys used for turbine bolting purposes should be as close as possibleto the coefficient of thermal expansion of the metal from which theturbine shell is formed. In the double aged condition above described,Alloy 2 manifested a coefficient of thermal expansion, after heating to1,000 F. and holding for minutes, of 7.8 in./in./ F. This value comparesquite favorably with the value of 7.75 l0" to 8 l0 in./in./ F. discussedpreviously herein.

In an effort to determine stress relaxation characteristics, alloy 2 wascompared against an alloy known to exhibit satisfactory resistance torelaxation as a bolting material. In this connection, three separatedeterminations were made using three different strain values. Theinitial strain was 0.15 percent which was maintained over the fullcourse (1,000 hours) of the first test. The stress at the beginning ofthis test was about 38,080 p.s.i., the temperature being 1,1 12 F. (sametemperature used in all three tests). After exposure for 1,000 hours,the final stress value was then determined, a level of approximately30,200 p.s.i. being obtained. This value was virtually identical withthat manifested by the standard alloy of comparison under the sameconditions of test.

In the second run the strain was increased to and maintained at 0.257percent, the load in this instance being initially about 65,000 p.s.i.After approximately 144 hours, the stress was determined to be 56,100p.s.i. 1n the last experiment the strain was maintained at 0.30 percent,the initial stress being 75,600 p.s.i. After nearly 170 hours, thestress was measured to be 65,500 p.s.i. As with the first, the secondand third tests indicated that alloy 2 compared quite favorably with thestandard alloy. These data, although ascertained by way of simulatedtest conditions, indicate that alloys contemplated within the inventionwill afford a more than satisfactory degree of resistance to stressrelaxation.

Alloys of the subject invention can be produced in accordance with usualand conventional processing techniques as already indicated and as thoseskilled in the art will readily appreciate. it is preferred that vacuuminduction techniques be employed although the alloys can be readily airmelted. After forming ingots and prior to hot working, the ingots shouldbe thoroughly homogenized at, say, a temperature on the order of about2,100 F. This contributes to achieving uniform distribution of thealloying constituents and also better mechanical properties. The castingots can be initially hammered or press forged and subsequently hotrolled or the ingots can be hot rolled directly to plate or sheet withsuitable intervening reheat treatments in order to maintain thetemperature above about l,700 F. Where used, annealing treatments shouldbe conducted within the temperature range of approximately 1,750 F. tol,850 F. as opposed to higher temperatures. It has been found that thelower annealing temperatures confer higher strength characteristics.

The alloys of the present invention can be produced in the form of bar,rod, sheet, plate, extruded tubing, and forgings and are useful atelevated temperatures on the order of about l,000 F. for suchapplications as steam piping, tubing, etc. Of course, the alloys areparticularly adapted for use as fasteners in steam turbine assemblies,particularly bolting for fastening the outer shells or casings (usuallyflanged) of such assemblies. This follows from the excellent minimumyield Strengths (0.02 percent offset) afiorded at both room temperatureand at 1.000 F. in combination with other desired characteristicsdiscussed herein. (The aging treatment criterion used in determining theminimum yield strength is 24 hours at l,300 F. followed by air cooling.)

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium alloy adapted for use at elevated temperatures onthe order of about l,000 F. and characterized in having a yield strengthat room temperature of at least about 85,000 p.s.i. and a yield strengthat l,000 F. of at least about 70,000 p.s.i. together with a high degreeof resistance to notch sensitivity, said alloy consisting essentially ofabout 17.5 percent to 22 percent chromium, about 2.3 percent to 3.3percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5percent to 3.25 percent tungsten, about 0.4 percent to 0.75 aluminum,from 0.35 percent to 0.7 titanium, the sum of the aluminum plus titaniumat least 0.9 percent and up to 1.4 percent, the aluminum, titaniumcolumbium, molybdenum, and tungsten being correlated such that thestrengthening and stability factor, SSF, expressed by the followingrelationship is satisfied 2.2 %Al+l.2 %Ti-i-O.6X%Cb+ about 5.25% to6.4%, about 0.01 percent to 0.12 percent carbon, about 3 percent to 12percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium, upto 0.4 percent silicon, up to 0.75 percent manganese, and the balanceessentially nickel.

2. An alloy in accordance with claim 1 in which the chromium is from 19percent the aluminum plus titanium is at least 1 percent the SSF valueis from 5.4 percent to 6.2 percent, the sum of the columbium plusmolybdenum does not exceed about 6 percent, and the carbon is from 0.04percent to 0.1 percent.

3. In a steam turbine assembly, a fastener for bolting the outer shellsections thereof and formed from an alloy consisting essentially of atleast 14 percent to 22 percent chromium, about 2.3 percent to 3.3percent columbium, about 2.5 percent to 3 percent molybdenum, about 2.5percent to 3.25 percent tungsten, about 0.4 percent to 0.75 percentaluminum, from 0.35 percent to 0.7 percent titanium, the aluminum,titanium, columbium, molybdenum and tungsten being correlated such thatthe strengthening and stability factor, SSF, expressed by the followingrelationship is satisfied about 5.25% to 6.4%, with the further provisosthat when the chromium content is less than 17.5 percent (a) thealuminum content is at least 0.5 percent, (b) the sum of the aluminumplus titanium is at least 1.1 percent and (c) the SSF is at least 5.5percent, about 0.01 percent to 0.12 percent carbon, about 3 percent to12 percent iron, up to 0.01 percent boron, up to 0.1 percent zirconium,up to 0.4 percent silicon, up to 0.75 percent manganese, and the balanceessentially nickel, said alloy being further characterized in having ayield strength at room temperature of at least about 85,000 p.s.i. and ayield strength at l,000 F. of at least about 70,000 p.s.i.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 9 ,l83Dated November 1971 JOHN H. OLSON and JERE H. BROPHY Inventor(s) It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 40, for "10" read ----further-; line 43, for

"7.75 x 10 to s x 10 read --7.75 x 10" to s x 10 line 46, for "8.4 X 10read 8.4 x l0 and line 52, for "difficulty" read --difficultly-.

Column 4, line 6, for "1300+F." read l300F.-; line 9,

for "temperature" read -temperatures;1ine 63, for "later" read -latter-;and line 71, for "l,800,F. read l,800F.-.

Column 5 TABLE I, under column heading "Cb" for Alloy No. 3

for "391" read --3.l-; under column heading "C" Alloy No. 6, for "0.72"read 0.072; for column heading "S.s.f. factor" read -S.S.F. factor;under column heading "S.S.F. factor", Alloy No. A, for "511" read 5.ll;and under column heading "C", Alloy No. C, for "0.84" read 0.084-.

Column 6 TABLE III for column heading g- 5%" read NTS and line 75, for"25.20 F." read --25F.. Column 7 line 32 for "7.75 x 10 to 8 x 10 read--7.75 X 10 to 8 x 10' Column 8, line 32 (Claim 1, line 10) before"aluminum insert --percent--; line 33 (Claim 1, line 11) after"titanium" insert -being--; and line 61 (Claim 3, line 11) for "%C+"read %Cb.

Signed and sealed this 6th day of June 1972. L .J

(SEAL) Attest:

EDWARD M.FLET( JHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents

2. An alloy in accordance with claim 1 in which the chromium is from 19percent the aluminum plus titanium is at least 1 percent the SSF valueis from 5.4 percent to 6.2 percent, the sum of the columbium plusmolybdenum does not exceed about 6 percent, and the carbon is from 0.04percent to 0.1 percent.
 3. In a steam turbine assembly, a fastener forbolting the outer shell sections thereof and formed from an alloyconsisting essentially of at least 14 percent to 22 percent chromium,about 2.3 percent to 3.3 Percent columbium, about 2.5 percent to 3percent molybdenum, about 2.5 percent to 3.25 percent tungsten, about0.4 percent to 0.75 percent aluminum, from 0.35 percent to 0.7 percenttitanium, the aluminum, titanium, columbium, molybdenum and tungstenbeing correlated such that the strengthening and stability factor, SSF,expressed by the following relationship is satisfied 2.2 X %A1+ 1.2 X%Ti+ 0.6 X % Cb+ 0.6 X %Mo+ 0.3 X %W- 5 X %C is from about 5.25% to6.4%, with the further provisos that when the chromium content is lessthan 17.5 percent (a) the aluminum content is at least 0.5 percent, (b)the sum of the aluminum plus titanium is at least 1.1 percent and (c)the SSF is at least 5.5 percent, about 0.01 percent to 0.12 percentcarbon, about 3 percent to 12 percent iron, up to 0.01 percent boron, upto 0.1 percent zirconium, up to 0.4 percent silicon, up to 0.75 percentmanganese, and the balance essentially nickel, said alloy being furthercharacterized in having a yield strength at room temperature of at leastabout 85,000 p.s.i. and a yield strength at 1,000* F. of at least about70,000 p.s.i.